Codec based on AMR (Adaptive Multi-Rate) as one of voice compression and coding systems widely used for cellular phones is designed to, when there exists no voice signal on a transmission side, transmit pseudo noise information whose data volume is smaller than that of the voice signal without transmitting the voice signal to a reception side, thereby reducing power consumption at the time of a call over a cellular phone.
In this case, the reception side generates a pseudo noise by using the received pseudo noise information and outputs the noise through a call receiver unit (speaker), thereby mitigating a sense of interruption of a call.
FIG. 7 is a block diagram showing an electrical structure of a main part of a cellular phone of this kind.
The cellular phone 1 has a foldable casing formed of an upper unit 10 and a lower unit 20 as shown in FIG. 7. Accommodated in the upper unit 10 are a display unit 11, an antenna 12, a call receiver unit (speaker) 13, a receiving call/charging lamp 14 and a magnet 15. The antenna 12 transmits and receives radio waves to/from a radio base station not shown. The radio waves include a signal transmitted from a cellular phone on the transmission side not shown. The signal is coded based on AMR, one frame of which includes frame type information indicating whether the frame is a voice signal or pseudo noise information corresponding to a case where there exists no such a voice signal. The display unit 11, which is formed of, for example, an LCD (Liquid Crystal Display), displays various pieces of information such as calling party telephone number information and icon information. The call receiver unit 13 generates a voice of a transmission partner and transfers the same to a user. The receiving call/charging lamp 14 blinks in blue when a call arrives and lights in red when in charging. The magnet 15 irradiates magnetism to the lower unit 20 when the upper unit 10 and the lower unit 20 are brought to be closed.
Accommodated in the lower unit 20 are an operation unit 21, a microphone 22, an RF circuit 23, a modulation/demodulation circuit 24, a baseband processing circuit 25, a codec circuit 26, a magnetic sensor 27, a storage unit 28 and a control unit 30. The operation unit 21 is formed of a transmission button, English characters/Japanese syllables/Chinese characters used in Japanese writing/numerals conversion buttons, a power on/off button, a cross button for cursor operation and an end button. The microphone 22 transmits a call upon receiving a user's voice. The RF circuit 23 has a reception circuit, a transmission circuit and a frequency synthesizer not shown.
The modulation/demodulation circuit 24 executes demodulation of received radio waves and modulation of radio waves to be transmitted. The baseband processing circuit 25 takes out an original baseband signal from a demodulation signal output from the modulation/demodulation circuit 24 and supplies the same to the codec circuit 26, as well as taking character data from the demodulation signal and supplying the same to the control unit 30. The codec circuit 26 executes digital/analog conversion (hereinafter, referred to as “D/A conversion”) of a baseband signal output from the baseband processing circuit 25 and supplies the obtained signal to the call receiver unit 14, as well as executing D/A conversion of an output signal of the control unit 30. The magnetic sensor 27, which is formed of, for example, Hall elements, generates a magnetism detection signal M when the upper unit 10 and the lower unit 20 are brought to be open to prevent irradiation of magnetism of the magnet 15. The storage unit 28 stores a control program for operating the control unit 30 and data to be displayed on the display unit 11. The control unit 30, which is formed, for example, of a CPU (Central Processing Device), comprises an open state detection unit 31, a line control unit 32 and a display control unit 33 to control the entire cellular phone 1. The open state detection unit 31 detects the magnetism detection signal M of the magnetic sensor 27 to generate an open state detection signal N when the upper unit 10 and the lower unit 20 are brought to be open. The display control unit 33, which is formed of, for example, an LCD driver, drives the display unit 11.
FIG. 8 is a block diagram showing an electrical structure of a main part of the codec circuit 26 in FIG. 7.
The codec circuit 26 has an AMR decoder 26a with an internal pseudo noise generator and a D/A conversion unit 26b. The AMR decoder 26a with an internal pseudo noise generator, when the above-described frame type information included in a baseband signal “in” which is output from the baseband processing circuit 25 represents a voice signal, decodes the voice signal and outputs the obtained signal as a signal e, while when the frame type information represents pseudo noise information, generating a pseudo noise signal corresponding to the pseudo noise information and outputs the obtained signal as the signal e. The D/A conversion unit 26b D/A converts the signal e from the AMR decoder 26a with an internal pseudo noise generator and sends the converted signal as a signal f to the call receiver unit 13.
Among other techniques of this kind than the above-described cellular phone is such a techniques as recited in the following literature.
The voice coding communication system recited in Literature 1, for example, resolves the problem in a sense of hearing because while a transmission side stops transmission during a non-voice period, a pseudo noise generated on a reception side attains approximately the same voice quality and level as those of a background noise sent from the transmission side.
Literature 1: Japanese Patent Laying-Open No. 05-083208 (page 6, FIG. 1)
The above-described cellular phone has the following problems.
More specifically, although from a moment when determination is made that there exists no voice signal for transmission, a cellular phone on a transmission side calculates a pseudo noise level on the transmission side, there is a possibility that a frame in which a voice signal exists will be erroneously determined to be a frame having no voice signal existing. In this case, because a high pseudo noise level is calculated, a level of a pseudo noise signal generated by a cellar phone on a reception side becomes higher than that required, so that a pseudo noise unnatural in terms of a sense of hearing is generated. In general, a lower pseudo noise level is better as long as a user fails to feel interruption of a call. It is therefore unnecessary to faithfully reproduce, on the reception side, a pseudo noise level calculated on the transmission side.
While the voice coding communication system recited in the above-described Literature 1 has a similar object of resolving problems in a sense of hearing, it has a different structure with a pseudo noise on the reception side set to be approximately the same as a background noise on the transmission side.
The present invention, in view of the above-described circumstances, aims to provide a cellular phone which generates a pseudo noise whose level is low within a range in which a user fails to feel a sense of interruption of a call, and a codec circuit and a receiving call sound volume automatic adjustment method for use in the cellular phone.